The present invention relates to capacitive energy storage devices for use at cryogenic temperatures, and in particular to the use of ferroelectric ceramic materials in such capacitive storage devices.
Storing energy in banks of capacitors at room temperature is commonly used in applications where size or weight has not been a major concern. Such capacitor technology is quite advanced. However, where the size and weight of the capacitive energy storage devices are of significance and are desired to be minimized, and where rapid discharge is desired, there are potential benefits to be obtained by capacitive energy storage at cryogenic temperatures.
These potential benefits are due to the fact that dielectric breakdown field strengths of dielectric materials used in capacitors are generally much larger at low temperatures than at room temperatures. Also, the dissipation factor for such dielectric materials generally decreases with decreasing temperature so that dielectric heating is reduced in charge-discharge operations. The dissipation factor is a measure of internal power losses due to electronic conduction through the dielectric. This power loss results in the thermal dissipation of electrical energy which is undesirable because it raises the temperature of the device and degrades its efficiency. Finally, the resistivity of metals falls rapidly with decreasing temperature so that Joule heating in metal components is reduced during discharge at cryogenic temperatures. In capacitive energy storage devices, the Helmholtz free energy density of the dielectric is an important quantity. The larger the Helmholtz free energy density of the dielectric, the greater the energy per unit volume which can be stored. The Helmholtz free energy density is defined by the following equation: ##EQU1## where F is the Helmholtz free energy, is the dielectric constant of the material, E is the electric field strength, and E.sub.c is the upper limit of electric field strength.
Some studies of capacitive energy storage at cryogenic temperatures have been published. One study dealt with the impregnation of dielectric films with liquid nitrogen or polar liquids. K. N. Mathes and S. H. Minnich, "Cryogenic Capacitor Investigation," Final Report, S-67-1095, May 1965. Three types of materials were investigated at 77 K, and it was concluded that energy densities of approximately 0.6 J/cm.sup.3 were possible. Energy density may be defined as the energy per unit volume of a medium.
The use of strontium titanate glass ceramic materials as capacitive energy storage devices at cryogenic temperatures was reported by Lawless, Proc. XIII Int'l. Congress of Refrigeration, Washington, D.C., 1971, Vol. 1, p. 599. Based on measurements of electric field strength and dielectric breakdown at 77 K, it was concluded that energy densities of approximately 5.0 J/cm.sup.3 were possible.
However, there is a need in the art for materials which can be used as capacitive energy storage devices and which have even greater energy densities. The size and weight of capacitive energy storage devices could be reduced providing portability to devices which have been heretofore too large and bulky to be mobile. For example, high powered lasers require massive capacitor banks which are too large and heavy to be moved easily. Capacitive devices having large energy densities could reduce the necessary bulk of the capacitors presently utilized in such applications.